The invention relates to a method for production of a hollow vessel, particularly a container drum, having at least one encircling projection on the periphery of the hollow vessel. Such a hollow object or drum is made from a tubular blank which is extruded into an open mold and then blow formed in the mold once it has been closed. The encircling projection is formed directly out of the hot-formable drum material during the blow molding process by a correspondingly shaped mold slide, displaceably guided in the blowing mold.
Such projections generally have a definite cross sectional shape, profiled to serve their purpose in subsequent use, for example, as rolling treads on drums, or to provide a purchase for cranes and other handling means. If such projections are formed out of the unreinforced wall material of the blank, defects in the material can result because the blank is blown into the annular recess which forms the outward projection. The material entering and adjoining the annular recess is stretched out and thereby thinned. Subsequently, when the slide is closed, the material is folded and welded because it is still in a hot-formable condition. Weakening indentations can occur in the welding seam, creating a risk of rupture under hydraulic pressure if a filled drum is dropped.
In attempts to alleviate this problem the shell material has been thickened in the area of the projection so that there is sufficient upsetting material available in the final forming operation. This thickening of the shell material does provide some security, however, during the upsetting of the material the welding process results in one or two outward-directed indentations in the shell of the vessel. Stress concentrations are formed by these indentations when the drum is loaded and may start a tear in the shell. Various methods have been tried to counteract these indentations by adding material to the areas of indentation.
Thus, one attempt has been to provide an encircling increase in thickness of the tube blank only in the immediate area of the recess in the mold. When the tube blank is inflated during the blowing process the projecting thickened area is intended to enter the annular recess in the mold, so that the annular projection on the drum will be molded exclusively out of the added projecting material. This method of forming, out of an added bulge exclusively, suggests that no folding of the material and no welding of folds occurs.
However, because the tube blank is produced by extrusion, the desired effect has not been achieved because encircling increase in thickness of the material must be produced by orifice control and consequently by the addition of material. When, for example, a sharply defined and narrow encircling bulge is formed by the addition of material the bulge will tend to drift out of the horizontal plane in the zone of the weld seam and undulate over the periphery of the tube during the blowing operation.
During the blowing operation, the least stretching of the tube blank takes place in the zone of the weld seam, and hence the material is the thickest in this region. If, for example, two rings are provided, the curvatures of the two will be opposed to each other and they cannot be straightened out by orifice control. If the areas reinforced by added material enter the annular mold recess at all, they do so only partially. A satisfactory blown ring is then unattainable.
An objective of the present invention is therefore to correct the indentation of the welding seam of the molded projection on the interior surface of the vessel by smoothing the indentation to an extent sufficient to eliminate the undesirable stress concentrations.